Fasteners such as nuts and bolts may hold together elements of a machine.
A nut, for example, may become loose, which may disadvantageously cause elements of the machine to become separated. Such separation may interfere with operation of the machine or cause damage to the machine or to persons or property near the machine. Known apparatuses for determining whether a nut has become loose include
Apparatuses that change colour when a nut has become loose, and sets of arrows or tabs that may be aligned when the nut is tight but become misaligned when the nut has become loose. Such apparatus require visual inspection to determine that a nut is loose, and therefore frequent visual inspection is required. If visual inspection is not sufficiently frequent, a fastener may become loose and thereby cause damage.
In another example of a device for indicating that a nut has become loose (U.S. Pat. No. 4,709,654), movement of the nut relative to an axle stud causes mechanical movement of an indicator under the force of a spring. The indicator moves from a retracted position and protrudes beyond the outer surface of the nut, and can be seen or felt during an inspection. Detection of the loose nut requires visible inspection or inspection by hand.
In the particular case of wheel nuts, looseness of the nuts can be detected by sensing knocking associated with an axle and a wheel hub assembly (U.S. Pat. No. 5,959,365). Signals from the sensors can be used to generate alerts to the driver, for example.
Another known approach is to use a sensor assembly secured to the mounting hub and with a pointer in contact with the wheel, which detects relative motion between the hub and the wheel (U.S. Pat. No. 7,012,511). The housing of the sensor assembly may be made from piezo-ceramic material which generates an electric current proportional to the amount of the deflection.
A pressure sensor may be used to measure the torque relative to a desired value. An example sensor is a piezo-resistive pressure sensor which measures pressure by a change in conductivity of an integrated semiconductor. Another example approach is to use a Hall-effect sensor and a corresponding magnet. Wireless communications may be used between the sensor and a control module (U.S. Pat. No. 7,994,901).
In other applications such as vibration testing, a strain gauge may be used to measure the looseness of nuts tightened onto bolts and shaken loose by vibration (U.S. Pat. No. 4,862,746).